Hybrid Modulation Method for Nearest-Level-Control-Based MMC to Suppress DC Power Fluctuation When Enabling Circulating Current Suppression

Circulating currents in the modular multilevel converter (MMC) will increase the rms value of arm current and thus reduce the efficiency SMs, as a result, circulating current suppression control (CCSC) is widely used in MMC system. For MMC using conventional nearest level control (NLC) with dozens of submodules per arm, CCSC will introduce fluctuations in the DC current/power delivered by MMC. This paper comprehensively analyzes this mechanism of the DC current distortion, and a hybrid modulation method which combine the advantages of NLC and PWM is proposed to solve this problem, together with the capacitor voltage balancing strategy.